Super alloy ionization chamber for reactive samples

ABSTRACT

The present invention relates to an ionization source having a chamber for ionizing a sample. The ionization chamber has surfaces to reduce the overall interaction with reactive samples. The inner surface walls of the ionization chamber or the ionization chamber may be formed from an inert super alloy. For instance, Inconel™ 625, Inconel™ 601 or Hastelloy® may comprise the ionization chamber or the surface walls of the ionization chamber. The invention also includes a method for reducing the interaction of a reactive analyte with an ion source.

CROSS-REFERENCE

This application is a divisional of Ser. No. 10/810,396, filed on Mar.25, 2004, now U.S. Pat. No. 6,974,956 which application is a divisionalof Ser. No. 09/894,204, filed Jun. 28, 2001, now U.S. Pat. No.6,765,215.

TECHNICAL FIELD

This invention relates generally to ion source chambers for use in massspectrometry. More particularly, the invention relates to an ionizationchamber made of a “super alloy” that provides reduced interaction withreactive samples.

BACKGROUND

Typical mass spectrometers contain an ion source having an ionizationchamber. A sample containing an analyte is introduced into theionization chamber through a means for sample introduction. Once theanalyte is disposed within the ionization chamber, an ionization sourceproduces ions from the sample. The resultant ions are then processed byat least one analyzer or filter that separates the ions according totheir mass-to-charge ratio. The ions are collected in a detector, whichmeasures the number and distribution of the ions, and a data processingsystem uses the measurements from the detector to produce the massspectrum of the analyte. The sample can be in gaseous form or, dependingupon the particular analyte separation and ionization means, caninitially be a component of a liquid or gel.

There are many types of ion sources that are useful in mass spectrometry(hereinafter referred to as MS). Sources of ionization sources include,but are not limited to, electron impact, chemical ionization, plasma,fast ion or atom bombardment, field desorption, laser desorption, plasmadesorption, thermospray and electrospray. Two of the most widely usedsources for gaseous analytes are the electron impact (hereinafterreferred to as EI) and chemical ionization (hereinafter referred to asCI) sources.

An EI source generally contains a heated filament giving off electronsthat are accelerated toward an anode and collide with gaseous analytemolecules introduced into the ionization chamber. Typically, theelectrons have energies of about 70 eV and produce ions with anefficiency of less than a few percent. This energy is typically chosenbecause it is well in excess of the minimum energy required to ionizeand fragment molecules and is at or near the peak of the ionizationefficiency curve for most molecules. The total pressure within theionization source is normally held at less than about 10⁻³ torr. Theions produced are extracted from the EI source with an applied electricfield and introduced into an analyzer wherein they are separated bymass-to-charge ratio. The selected ions are registered as an ion currentcharacteristic of the specified mass/charge by the ion detection andsignal processing system of the mass spectrometer. Those ions ideally donot collide with other molecules or surfaces from the time they areformed in the EI source until the time they are collected in thedetector. An EI source is often employed in MS in conjunction with gaschromatography (GC), which separates constituents of the analyte by timeof elution.

The EI ion source is often used with a quadrupole mass spectrometer forreasons of stability and reproducibility of ion-fragmentation patterns.The patterns produced can be compared with “classical” spectra librariesand the ion's molecular composition thereby determined. Thus the qualityof the spectral pattern produced by the ion source may greatly effectthe interpretation of data.

In EI, the character and quantity of analyzable ions from the moleculesin the sample depend upon reactions occurring on the inner surfaces ofthe chamber containing the source of ionization. First, the analyte isintroduced into an ionization chamber wherein ionization of the analyteis intended. Before ionization, however, much of the sample is exposedto inner surfaces of the chamber, which are usually heated. Theinteraction of the sample with these surfaces may create an undesiredeffect. For example, if a portion of the sample adheres to the chambersurface, the portion cannot be effectively ionized and directed to thedetector. As a result, the sensitivity of the apparatus for analysis ofthat analyte may suffer. In addition, the sample can degrade, i.e.,convert to other compounds or be adsorbed onto the surface of thechamber and desorb later. Depending upon the compound, many unexpectedions can appear as a result of the interaction of the compound with thesurfaces. The results are undesirable: chromatographic peak tailing,loss of sensitivity, nonlinearity, erratic performance and the like.

In addition, cleanliness is critical to the proper performance of themass spectrometer using an EI source, particularly for quantitativeanalysis of material in a low concentration, such as for GC/MS analysisof pesticide residues, drug residues and metabolites, environmentalsamples and trace analysis of organic compounds. The relativelynon-volatile materials in the sample matrix generally form insulatingdeposits on the surface of the chamber that take on an electricalcharge. This charge distorts the applied electric field causinganomalies in ion production. Often, abrasive cleaning is employed toensure that the chamber is substantially free of insulating deposits.

In contrast to the EI ion source, a CI source produces ions throughcollision of the molecules in the analyte with primary ions present inthe ionization chamber or by attachment of low energy electrons presentin the chamber. A CI source operates at much higher pressures than an EIsource in order to permit frequent collisions. The overall pressure in aCI source during operation typically ranges from about 0.1 to about 2torr. This pressure may be produced by the flow of a reagent gas, suchas methane, isobutane, ammonia or the like, that is pumped into thechamber containing the CI source. In a typical configuration, both thereagent gas and the analyte are introduced through gas-tight seals intothe chamber containing the CI source. The reagent gas and the analyteare sprayed with electrons having energies of 50 to 300 eV from afilament through a small orifice, generally less than 1 mm in diameter.Ions formed are extracted through another small orifice, also generallyless than 1 mm in diameter, and introduced into the analyzer or filter.Electric fields may be applied inside the CI source, but they areusually not necessary for operation of the CI source. Ions eventuallyleave the CI source through a combination of diffusion and entrainmentin the flow of the reagent gas. Thus, it is evident that CI sourcesoperate in a substantially different manner from EI sources. However,the same undesired interactions of the sample with the source chambersurfaces may occur in a CI source as in an EI source as mentioned above.

Efforts have been made to address sample degradation problems in theionization chamber of a mass spectrometer, particularly those containingan EI ion source, by substituting for or modifying the surfaces of theionization chamber. Such efforts include providing a metallic surfacewith advantageous properties. For example, ionization chambers have beenmade with electropolished stainless steel surfaces in efforts to reducethe total active surface area. However, mass spectrometers using suchionization chambers have been found to give variable results and stillexhibit degradation of the analyte over time. U.S. Pat. No. 5,055,678 toTaylor et al. describes the use of a chromium or oxidized chromiumsurface in a sample analyzing and ionizing apparatus, such as an iontrap or EI ionization chamber, to prevent degradation or decompositionof a sample in contact with the surface. This reference also describesthat coating the inner surface of the ionization chamber with materialsknown for corrosion resistance or inertness, such as gold, nickel andrhodium, may reduce degradation of analytes, such as pesticides, drugsand metabolites, to some degree. Such surfaces suffer from a variety ofdrawbacks such as susceptibility to scratching when the metal coating issoft or assembly/disassembly difficulties when the coating has a highcoefficient of friction.

In addition, U.S. Pat. No. 5,633,497 to Brittain et al. describes theuse of a thin coating of an inert, inorganic non-metallic insulator orsemiconductor material on the interior surfaces of an ion trap or EIionization chamber to reduce adsorption, degradation or decomposition ofa sample contacting the chamber surface. The material disclosed in thisreference was fused silica, with aluminum oxide, silicon nitride and“selected semiconductors” given as alternative embodiments. Becausethese surface coatings exhibit high electrical resistivity, however,electrical charge can undesirably accumulate on these coatings if thecoatings are too thick. The important feature of the invention describedin this reference is the use of a sufficiently thin coating of insulatorthat charging effects do not occur.

U.S. Pat. No. 5,796,100 to Palermo discloses a quadrupole ion traphaving inner surfaces formed from molybdenum.

In addition, U.S. Pat. No. 6,037,587 to Dowell et al. describes a massspectrometer having a CI source containing a chemical ionization chamberhaving inner surfaces formed from molybdenum.

Others have attempted to prevent degradation problems by treating theinner metal surfaces of the analytical apparatus with a passivatingagent to mask or destroy active surface sites. For example,alkylchlorosilanes and other silanizing agents have been used to treatinjectors, chromatographic columns, transfer lines and detectors in GC.See, e.g., U.S. Pat. No. 4,999,162 to Wells et al. Such treatments havebeen successful in deactivating metal surfaces and thus have preventeddegradation of some species of analyte. Unfortunately, the materialsused for such treatments have a sufficiently high vapor pressure tointroduce organic materials in the gas phase within the volume of theionization chamber that are ionized along with the analyte, producing ahigh chemical background in the mass spectrum.

One problem with the described techniques is that each of the metals isapplied as a coating. Over time, the coatings will begin to wear ortarnish. It, therefore, would be desirable to be able to construct anionization chamber for reactive samples that employs a single metal oralloy. This provides for longer wear as well as makes the manufacture ofthe parts less complicated.

To date, there are very few patents or inventions related to the use of“super alloys” with mass spectrometer parts. For instance, U.S. Pat. No.6,025,591, discloses the use of a nickel coating on glass fibers to makeconducting surfaces for the quadrupole of a mass spectrometer.Furthermore, U.S. Pat. No. 5,625,185 claims the use of graphite forentrance lenses of the ion optics of an ICP-MS system to avoidsputtering effects seen when using normal steels containing iron,nickel, and chromium.

Thus, there is a need to reduce the adsorption, degradation anddecomposition of these important analyte ions in an ionization chamberand to mitigate the problems associated with known coatings.

SUMMARY OF THE INVENTION

The invention provides an apparatus and method for improved sampleionization within an ionization chamber of a mass spectrometer. Theionization chamber comprises an inert super alloy such as Inconel™ 625,Inconel™ 601, or Hastelloy®. The super alloy may also be applied as acoating to the ionization chamber. The super alloy contains a low ironcontent that provides for reduced absorption, decomposition and reactionof the samples with the ionization chamber.

BRIEF DESCRIPTION OF THE FIGURES

The invention is described in detail below with reference to thefollowing figures:

FIG. 1A is a simplified diagrammatic sketch in a section view showing afirst embodiment of the mass spectrometer containing an EI chamber thatincorporates the invention.

FIG. 1B is a simplified diagrammatic sketch in a section view showing asecond embodiment of the mass spectrometer containing an EI chamber thatincorporates the invention.

FIG. 2 is a table that compares relative response factors (RRFs) of2,4-dinitrophenol for the following ion source materials: stainlesssteel and Inconel™ 625.

DETAILED DESCRIPTION OF THE INVENTION

Before describing the invention in detail, it must be noted that, asused in this specification and the appended claims, the singular forms“a,” “an,” and “the” include plural referents unless the context clearlydictates otherwise. Thus, for example, reference to “a material”includes combinations of materials, reference to “a compound” includesmixtures of compounds, reference to “an ion source” includes more thanone ion source, reference to “a chamber” includes a plurality ofchambers, and the like.

In describing and claiming the present invention, the followingterminology will be used in accordance with the definitions set outbelow.

The term “ionization chamber” refers to solid structure thatsubstantially encloses a volume in which the sample, typically a gas, isionized. The solid structure may also constitute part of a massanalyzer; for example, an ion trap wherein electron impact or chemicalionization occurs inside the trap.

The term “inner surface” refers to any surface within the chamber thatcan be subject to undesirable interaction with the analyte. The termencompasses surfaces of a component that may not be a part of thechamber, but that is disposed within the chamber, such as means forsample introduction.

The term “microstructure” is used herein to refer to a microscopicstructure of a material and encompasses concepts such as latticestructure, degrees of crystallinity, dislocations, grain boundaries andthe like.

The term “super alloy” refers to an alloy both inert and non-inert thatprovides resistance to abrasion and corrosion and that has a low ironcontent. A low iron content means an alloy having an iron content ofless than 25%. The super alloy contains 10–30% chromium and less than30% molybdenum. Nickel comprises at least 40% of the super alloy and isthe highest element percentage. Examples are materials having the namesInconel™ and Hastelloy®.

The term “Inconel™ 625” refers to a super alloy material comprising atleast 58% nickel, 20–23% chromium, 0.1% carbon, 0.5% manganese, 0.5%silicon, no more than 5.0% iron, no more than 0.015% sulfur, no copper,no more than 0.40% aluminum, no more than 0.40% titanium, no more than0.015% lead, no more than 1% cobalt, 3.15–4.15% niobium, no boron , and8.0–10.0% molybdenum.

The term “Inconel™ 601” refers to a super alloy material comprising58.0–63.0% Nickel, 21.0–25.0% Chromium, 1.0–1.7% aluminum, less than0.10% carbon, less than 1.0% manganese, less than 0.015% sulfur, lessthan 0.50% silicon, less than 1.0% copper and the remaining percentiron.

The term “Hastelloy®” refers to a super alloy material comprising 0–0.4%aluminum, 0–0.016% boron, 0–0.5% columbium and niobium, 1.5–5.0% cobalt,16–30% chromium, 0–2% copper, 3–20% iron, 0.5–1.5% manganese, 2.5–16%molybdenum, 43–71% nickel, 0.08–5% silicon, 0.07% or less titanium, 4%or less tungsten, 0.35% or less vanadium.

The invention is described herein with reference to the figures. Thefigures are not to scale, and in particular, certain dimensions may beexaggerated for clarity of presentation. FIG. 1A schematicallyillustrates a quadrupole mass spectrometer. Although the present exampleor diagram illustrates an EI source, the invention should not beconstrued narrowly to only this particular source and can be applied toother sources known in the art. An EI source 10 typically comprises anionization housing or substrate 11, a repeller electrode 12 and innersurfaces 13 that define a chamber 22 (See FIG. 1A). Housing or substrate11 as well as repeller electrode 12, may comprise any of the Inconel™625 and super alloy materials discussed below. In a second embodiment ofthe invention, inner surfaces 13′ may be applied as a coating tosubstrate or housing 11 (coating can be applied to all inner surfaces ofthe chamber as well as to the repeller electrode) (See FIG. 1B). Coating13′ may comprise any of super alloy and Inconel™ materials discussedbelow or incorporated by reference. In this embodiment of the invention,substrate or housing 11 may comprise an electrically-conductingmaterial. In the case of EI, the analyte gas 17 typically is introducedas a sample stream from a GC apparatus (not shown) into the chamberthrough an inlet orifice (not shown). An electron beam 15 that passesthrough orifices 19 into the chamber 22, from a filament 14 to anelectron collector 16, interacts with the analyte molecules 17 of theanalyte gas stream. The interaction results in formation of analyte ions18 that are repelled by the repeller electrode 12 that is charged to arepelling voltage with respect to the ions. The repelling voltage hasthe same polarity as that of the analyte ions. The repelling forcedrives the ions through a lens system 20 and a mass analyzer 30 thatselects the ions by mass-to-charge ratio. When the ions 18 reach thedetector system 40, their abundance is measured to produce a massspectrum for the sample. The quadrupole mass filter is preferred for theinvention, but various types of analyzers are also known in the art,e.g., ion traps, time-of-flight instruments and magnetic sectorspectrometers.

It has now been discovered that Inconel™ 625, Inconel™ 601, Hastelloy®and super alloys render surfaces within an ionization chamber more inertwith respect to certain known reactive analytes than typical chambersurface materials such as stainless steel, gold, nickel, chromium andchromium oxides, fused silica, aluminum oxide and molybdenum. Thosereactive analytes include, but are not limited to, acetophenone,2-acetylaminofluorene, 1-acetyl-2-thiourea, aldrin, 4-aminobiphenyl,aramite, barban, benzidine, benzoic acid, benzo(a)pyrene,1,4-dichlorobenzene, 2,4-dinitrophenol, hexachlorocyclopentadiene,4-nitrophenol, N-nitroso-di-n-propylamine and other compounds that occurin various solid waste matrices, soils, and water samples.

Alternatively, the super alloy may be employed as an inner surface foran ionization chamber. The super alloy may exhibit a layeredmicrostructure. Examples of super alloy compounds include, but are notlimited to Inconel™ 625, Inconel™ 601 and Hastelloy®. Surprisingly,these materials have also been found to be inert with respect to certainknown reactive analytes and to be hard and mechanically robust.

If the ionization chamber is coated with a dielectric, static chargewill accumulate on the dielectric during the ionization process. Suchcharging will cause arcing resulting in a false signal, or such chargedistribution may distort the field, thereby altering the ability of theionization chamber to produce ions. Thus, if an inert coating isemployed on any inner surface of the ionization chamber, it is preferredthat the coating is sufficiently electrically conductive to allowdissipation of charge, as disclosed below. Materials having a lowerresistivity may be deposited in a thicker coating on an inner surface ofthe ionization chamber. Irrespective of the resistivity of the coating,the coating should be uniformly deposited to insure that there are nouncoated areas or pinholes as well as to provide sufficient coverage tomask active sites on the surface. As is evident, any surface of theionization chamber, including the surfaces of the electrodes, is subjectto reaction with the uncharged reagent gas or the analyte.

There are many methods that can be employed to coat the compounds of thepresent invention onto the inner surface of an ionization chamber. Onemethod involves a two-step process: depositing a thin layer of a metalor alloy on the surface of interest and exposing the surface to anappropriate element under reaction conditions effective to form thedesired compound. There are many ways in which a thin layer of metal canbe deposited, e.g., by evaporation, sputtering, electroplating, chemicalvapor deposition (CVD), physical vapor deposition (PVD), etc, as isknown in the art. It is notable, though, that not all methods ofmetallic layer deposition can be employed with ease for any particularmetal. For example, a metal with a low melting point or boiling pointtemperature is particularly suitable for deposition through evaporation.Conversely, metals with a high melting point such as tungsten are noteasily deposited through evaporation. Once a layer of metal isdeposited, the layer can be exposed to a source of an appropriateelectronegative element under suitable conditions to form the desiredcompound. It is evident that proper film formation conditions mayinvolve high temperature processing; therefore, the material on whichthe surface is to be converted must be able to withstand all processingcondition.

Alternatively, the compounds of the present invention may be depositedon the surface in vacuum processes that do not involve two discretesteps as described above. Such vacuum processes include, but are notlimited to, cathodic arc PVD, electron-beam evaporation, enhanced arcPVD, CVD, magnetronic sputtering, molecular beam epitaxy, combinationsof such techniques and a variety of other techniques known to one ofordinary skill in the art. One of ordinary skill in the art willrecognize that CVD usually involves heating a substrate surface to asufficiently high temperature to decompose gaseous organic species toform the desired film. Such heating usually precludes the use of plasticas a surface on which the film is deposited. PVD, on the other hand,does not necessarily exclude plastics as a substrate and allows formasked film deposition. However, the method coats only surfaces that arewithin the “line of sight” of the source of the coating material, and“blind” spots are not coated. In addition, some substrate heating may beemployed in physical vapor deposition to promote film adhesion.

In addition, differences in thermal expansion coefficient between thecoating layer and the surface on which the coating is deposited can alsocontribute to adhesion problems if the surfaces are subject to drasticchanges in temperature.

The particular coating technique used generally affects themicrostructure, morphology, and other physical characteristics of thedeposited material. In addition, when the aforementioned depositiontechniques are employed, variations in processing parameters cansubstantially change the morphology of the deposited film. In general,it is desirable to produce a smooth film of generally uniform thickness.Smooth films tend to provide a lower surface area, thereby rendering thefilm kinetically unfavorable for reaction with analytes. Smoothness ofthe film will, however, be highly dependent on, and in generaldetermined by, the smoothness of the underlying surface.

As another alternative, the surface coating material can be applied as apowder. One method of powder application involves providing theconductive compound in powdered form and employing high pressure tospray the powder entrained in a fluid at high velocity such that thepowder mechanically adheres to the surface. Another method involvessuspending the powder in a solvent to form a paint, applying the paintonto the surface, and evaporating the solvent. The solvent can be arelatively inert carrier or one that facilitates chemical bondingbetween the powder particles or between the powder and the surface. Inaddition, heat can be applied to evaporate the solvent or to promotechemical bonding. Typically, no organic binder is used because organicmaterials generally outgas at sufficiently high vapor pressure toproduce a gas phase that is ionized along with the sample, producing ahigh background in the mass spectrum. However, the film of the presentinvention does not necessarily preclude inclusion of a small amount ofan organic binder if overall outgassing is sufficiently low. However,one drawback to this method is that the resulting coating does notwithstand abrasive cleaning as well and may have to be reapplied overtime.

Variations of the foregoing will be apparent to those of ordinary skillin the art. For example, while these coatings may be applied to surfacescomposed of stainless steel, such coatings can also be applied to othersurfaces such as aluminum or other structural materials that aretypically used to form an ionization chamber or other components of amass spectrometer. In addition, some compounds will be especially inertwith respect to some analytes, and a particular coating may be appliedto a surface that is designed for exposure to a specific analyte. Forexample, dinitrophenols are particularly reactive to components ofconventional mass spectrometers.

It is to be understood that while the invention has been described inconjunction with the preferred specific embodiments thereof, that theforegoing description as well as the examples that follow is intended toillustrate and not limit the scope of the invention. Other aspects,advantages and modifications within the scope of the invention will beapparent to those skilled in the art to which the invention pertains.

All patents, patent applications, and publications mentioned herein arehereby incorporated by reference in their entireties.

EXAMPLE 1

A freshly cleaned inner surface of a 316 stainless steel ionizationchamber was provided in an ion source of a mass spectrometer made byAgilent Technologies. The inner surface was cleaned by abrasion.Acenaphthene-d₁₀, a calibration standard, in a standard concentration,C_(is), of 40 ng/μL, was analyzed using the mass spectrometer. Theresponse of the mass spectrometer at mass 164 was used for the detectionof the acenaphthene-d₁₀. The analysis produced a peak area, A_(is), forthe internal standard. Then a series of analyte solutions were preparedthat contained 2,4-dinitrophenol in concentrations, C_(s), of 160, 120,80, 50, 20 and 10 ng/μL. The response of the mass spectrometer at mass184 was used for the detection of 2,4-dinitrophenol. Each solution wasanalyzed by the mass spectrometer, resulting in a series of peak areas,A_(s). For each solution, a relative response factor (RRF) wasdetermined according to the following equation:RRF=(A _(s) ×C _(is))/(A _(is) ×C _(s)).  (I)

The RRF for each solution is reported in FIG. 2. These RRFs provide astandard against which the inertness of the material is evaluated.

EXAMPLE 2

The parts forming the ionization chamber in Example 1 above werereplaced with parts of the same dimensions, but made of Inconel™ 625.The series of analyte solutions containing 2,4-dinitrophenol wasanalyzed in the mass spectrometer. For each solution, RRF was determinedaccording to equation (I). The RRF for each solution is reported in FIG.2. It is evident that for all concentrations of 2,4-dinitrophenol, RRFwas greater when parts were made of super alloys such as Inconel™ 625,Inconel™ 601, and Hastelloy®. This indicates that the super alloysurfaces are less reactive with respect to 2,4-dinitrophenol than afreshly cleaned 316 stainless steel surface with no coating.

1. A method of reducing interaction of a reactive analyte with a surfaceof a mass spectrometer ion source, comprising applying a coatingselected from the group consisting of Inconel™ 625, Inconel™ 601 andHastelloy® to the surface.